Set layer element id when building layers in PaintArtifactCompositor.

third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.cpp

Index: third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.cpp b/third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.cpp
index 8f2317a75fc1dba07ef262829306ce000d3fb0de..c20a0f0cfdafbb0b30d11b613b67d0758e711c92 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.cpp
+++ b/third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.cpp
@@ -29,6 +29,19 @@ bool isAncestorOf(const PropertyNode* ancestor, const PropertyNode* child) {
   return child == ancestor;
 }
 
+const CompositorElementId PropertyTreeState::compositorElementId() const {
+  // Zero or more of the scroll, effect or transform nodes could have a
+  // compositor element id. The order doesn't matter as the element id should be
+  // the same on all that have a non-default CompositorElementId.

[ajuma, 2017/01/20 15:07:46]

Is it worth DCHECK-ing that this holds?

[wkorman, 2017/01/20 22:13:56]

Done.

+  if (effect()->compositorElementId())
+    return effect()->compositorElementId();
+  if (scroll()->compositorElementId())
+    return scroll()->compositorElementId();
+  if (transform()->compositorElementId())
+    return transform()->compositorElementId();
+  return CompositorElementId();
+}
+
 PropertyTreeState::InnermostNode PropertyTreeState::innermostNode() const {
   // TODO(chrishtr): this is very inefficient when innermostNode() is called
   // repeatedly.